15min:

CHIH-HSUAN CHANG, GREGORY E. HALL AND TREVOR J. SEARS, Chemistry Department, Brookhaven National Laboratory, Upton, New York, 11973.

Saturation dip spectroscopy has been used to measure rovibronic transitions in the b ¹B₁ a ¹A₁ band system of CH₂ at sub-Doppler resolution. The radical was made by 308 nm excimer laser photolysis of a slowly flowing, low-pressure, sample of ketene (CH₂CO), optionally with added inert buffer gas. Typical observed linewidths in the pure precursor are approximately 8 MHz (FWHM), due to a combination of collisional lifetime and pump-probe beam crossing angle. Due to the non-zero ¹H proton nuclear spin, CH₂ exists as two distinct variants, ortho-CH₂ with I_H = 1 and para-CH₂ with I_H = 0. In ortho-CH₂, each rotational level consists of a triplet of hyperfine components corresponding to levels with F= J, J \pm 1. Most singlet CH₂ transitions show unresolved hyperfine structure in our experiment, since the largest splitting is due to I.J coupling, typically of the order of kHz. However, a small number of rotational levels in the v=0 level of the lower a state are known to be perturbed by accidentally near degenerate X ³B₁-state levels via spin-orbit coupling. Spectra involving such levels in ortho-CH₂ exhibit resolvable triplet, I.S, hyperfine splittings, with the splittings providing a direct measure of triplet state character of the level. We have measured hyperfine splittings for a number of pairs of perturbed levels confirming and refining previous estimates of the singlet-triplet mixing coefficients. Measurements of the pressure-dependent saturation recovery rates with different collision partners can give new insights into dephasing, velocity-changing and inelastic collisions relevant to pressure broadening and intersystem crossing mechanisms.

Acknowledgments: Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences.